Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a transmission line probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank.
An electromagnetic transmit signal is generated by a transceiver and propagated by the probe towards the surface of the product in the tank, and an electromagnetic reflection signal resulting from reflection of the transmit signal at the surface is propagated back towards to the transceiver.
Based on the transmit signal and the reflection signal, the distance to the surface of the product can be determined.
The transmit signal is typically not only reflected at the impedance transition constituted by the interface between the tank atmosphere and the surface of the product, but at several other impedance transitions encountered by the transmit signal. In the case of a GWR-system, one such impedance transition typically occurs at the connection between the transceiver and the probe. Generally, the transceiver is located outside the tank, and is connected to the probe via a feed-through going through a wall (typically the roof) of the tank.
Such a feed-through is typically formed by a coaxial line having the probe as its inner conductor, the tank wall or a connection piece that is attached to the tank as its outer conductor, and a dielectric member provided between the inner and outer conductors.
Due to the combined need for a sufficiently mechanically strong inner conductor and a practical outer conductor diameter, a feed-through impedance much above about 50Ω is seldom feasible. Hence, because of its structure, the impedance of the feed-through is generally similar to that of a typical coaxial cable, that is, about 50Ω.
A radar level gauge system is often mounted on a tubular mounting structure extending substantially vertically upwards from a roof of the tank. Such a mounting structure, which is often referred to as a “nozzle” may be a pipe that is welded to the tank and fitted with a flange at its upper end to allow attachment of an instrument, such as a radar level gauge system, or a blind flange. The inner diameter of the tubular mounting structure may typically be between 0.1 and 0.2 m, and a typical length may be around 0.5 m. In a tank arrangement comprising a tubular mounting structure (nozzle), the probe is typically mechanically connected to the tank at an upper end of the mounting structure, and passes through the mounting structure, past a lower end of the mounting structure, before entering the tank itself. At the upper end of the mounting structure, the probe may be electrically connected to the transceiver of the radar level gauge system through a feed-through that passes through the tank boundary.
For single conductor probes, sometimes also referred to as Goubau-probes, it has been found that propagation of the electromagnetic signal that is guided by the probe is affected by the tubular mounting structure, especially when the tubular mounting structure is relatively narrow and high.
Rather than having the properties of a surface waveguide, the single conductor probe inside the tubular mounting structure in effect acts like a coaxial transmission line with signal propagation properties depending on the dimensions of the tubular mounting structure. In particular, the impedance of the transmission line inside the tubular mounting structure may be in the order of 150Ω and may vary between installations. Accordingly, there will be a first impedance step at the interface between the feed-through and inside the tubular mounting structure and a second impedance step at the lower end of the tubular mounting structure.
The relatively large impedance step (about 150Ω to about 370Ω) at the lower end of the tubular mounting structure may disturb measurements of filling levels close to the lower end of the tubular mounting structure. In fact, the mismatch echo resulting from the above-mentioned impedance step may be stronger than the echo from an oil surface. In addition, multiple reflections between the impedance transition at the feed-through and the impedance transition at the lower end of the tubular mounting structure might lead to additional echo signals, which may disturb the filling level measurement relatively far below the lower end of the tubular mounting structure.
According to U.S. Pat. No. 6,690,320, problems caused by the reflection at the end of a tubular mounting structure are addressed by providing a coaxial cable extension inside the tubular mounting structure until after the probe exits the tubular mounting structure, so that the probe with the coaxial extension has the same impedance as the feed line between the transceiver and the probe (about 50Ω)). With this configuration, there is in effect no single conductor probe inside the tubular mounting structure, but the single conductor probe starts below the lower end of the tubular mounting structure, where the coaxial extension ends. As a result, there will be only one large impedance step, but since this impedance step is located below the lower end of the tubular mounting structure, the zone at the top of the tank where reliable filling level measurements cannot be performed (the so-called deadzone) will start at the end of the coaxial extension (below the tubular mounting structure), and will still be significant. Furthermore, there will be a significant loss of signal due to the strong reflection at the end of the coaxial extension, which limits the maximum measurable distance. Some signal will also find its way up in the tubular mounting structure (nozzle) and further disturb the echo situation.
To improve the above-described situation, EP 2 490 040 discloses a radar level gauge system comprising a single conductor probe and an impedance matching arrangement provided to the probe and extending along a portion of the probe. The impedance matching arrangement has a radial extension that may be constant or changing with a first rate of change inside the tubular mounting structure, and changes with increasing distance from the lower end of the tubular mounting structure with a second negative rate of change. The impedance matching arrangement (dielectric sleeve) decreases the impedance and locally reduces the radial extension of the electromagnetic field resulting from the transmitted electromagnetic signal within the tubular mounting structure, and then provides for a gradual change of the impedance from the impedance inside the tubular mounting structure to the impedance of the probe in the tank itself, below the lower end of the tubular mounting structure.
Although the solution according to EP 2 490 040 provides for an improvement in the determination of filling levels close to the tank ceiling and allows for measurement of filling levels in at least the lower part of the tubular mounting structure (nozzle), it would be desirable to provide for improved measurement of filling levels higher up in the nozzle.